The invention relates to an intensity modulation circuit for cathode-ray tubes, and in particular to an intensity modulation circuit capable of varying the brightness of the display screen of a cathode-ray tube only in a specified area.
Brightness gradation, as used herein, means a variation in the brightness among various portions of the screen. In monochrome television displays, for example, the screen can have several grades of brighter and darker areas on the screen. These variations, however, have no bearing on the average brightness of the screen. Dark images on the screen also have their own brightness grades as well as bright image.
Intensity modulation, as used herein means varying the average value of brightness only in a specified area on the screen by the use of an external rheostat, for example. Intensity modulation in monochrome television, for example, can be achieved by adjusting the rheostat to make the overall screen darker or brighter.
Reference is made to FIG. 1 wherein a conventional television receiver is depicted. The brightness on a cathode-ray tube is varied by changing a bias voltage between a first grid 152 and a cathode 150, as disclosed in Television Receiver From Principles to Practical Circuits, written by Tatsuo Tohmine and Masamichi Takano, and published by Keigaku Shuppan.
Reference is next made to FIG. 2 wherein the effect on intensity (brightness) of the variation in DC bias applied to grid 152 of the cathode-ray tube is depicted. The DC bias applied to grid 152 determines the operating point of the cathode-ray tube and the AC component of the video signal varies the grid potential across the operating point. Since the AC component of the video signal has an average voltage of zero, the average value of the grid potential is equal to the DC bias voltage. An average electrode beam (anode current) and a resulting intensity correspond to the bias voltage. Accordingly, it is the DC bias voltage which is responsible for providing an average brightness or intensity to the overall video image displayed on the television receiver.
Reference is next made to FIG. 3 wherein circuitry used to adjust the bias voltage between a first grid 162 and a cathode 161 is depicted. FIG. 3 depicts a conventional intensity modulation circuit. The positive voltage on cathode 161 is selected by movement of variable resistor 164 utilized for intensity modulation. Adjustment of variable resistor 164 to increase the bias voltage, between grid 162 and cathode 161, reduces the anode current, thereby making the screen darker.
The operating point varies between the cutoff point where the average intensity is zero and a maximum level which depends upon the display medium.
The prior art intensity modulation is achieved by varying the bias voltage between first grid and cathode of a cathode-ray tube. As is clearly seen in FIG. 2, intensity modulation is effected by the displacement of grid voltage along the x axis which causes a uniform variation in brightness over the entire screen. As a result, the brightness varies evenly over the screen, both in brighter and darker areas.
However, there is a need for the ability to vary the brightness of a television screen in only limited areas of the display. In addition, there is a need for an ability to select an area to be highlighted by a variation in intensity.